The bleaching pH plays a key role in ClO2 bleaching/brightening in the D1 and D2 stages. Our current understanding of optimum ClO2 bleaching pH is largely credited to the earlier work done by Raspon in 1956. Studying on Eastern Canadian softwood kraft pulp at kappa 28 with conventional chlorine based bleaching, Rapson showed an optimum D1 stage of 3.8 for maximum brightness. The maximum brightness corresponds to the minimum formation of two unproductive products, chlorite and chlorate, during ClO2 bleaching. Mill practice usually controls the D1 end pH at 3-3.5, a compromise between brightness development and dirt bleaching. In the absence of a dirt bleaching requirement, a mill usually controls the D2 pH at 4-4.5. Mills make no distinction between optimum bleaching pH requirements for SW or HW pulp. While these pHs are largely true for softwood pulp, the optimum bleaching pHs for hardwood species are much higher than 3.8 recommended by Rapson.